During limb skeletogenesis chondrocytes follow two developmental paths and produce permanent articular cartilage persisting through life or transient growth plate cartilage in which the cells mature, hypertrophy, and are replaced by bone. Though the critical importance of this developmental bifurcation for skeletal formation and function is widely recognized, the underlying mechanisms of regulation remain unclear, particularly at the molecular level. In the previous funding period, we focused on the transcription factor ERG and its variant C-1-1. Gain-of-function studies with chick and human C-1-1 show that: (a) C-1-1 is able to impose a stable and articular-like phenotype over the entire limb chondrocyte population, blocking growth plate formation, chondrocyte maturation and bone formation; (b) C-1-1 counteracts action of the pro-maturation transcription factor Runx2; and (c) the joint master regulator GDF-5 rapidly induces ERG/C-1-1 expression in developing synovial joints. These and other findings lead to our central hypotheses for this competitive continuation proposal: (a) C-1-1 acting down-stream of GDF-5 contributes to formation of permanent articular chondrocytes; and (b) C-1-1 in turn inhibits Runx2 function, maintains the permanent status of the cells, and prevents maturation and hypertrophy. Our aims are: (i) to functionally characterize murine ERG variants by cell and explant cultures and transgenic approaches;(ii) determine the consequences of conditional ERG gene ablation during development or postnatal life, using GDF-5-Cre and GDF-5-CreER mice; and (iii) determine the mechanisms by which GDF-5 triggers ERG expression by signaling pathways and promoter action, and how ERG/C-1-1 inhibits Runx2 function. The project will produce fundamental insights into genesis and function of articular chondrocytes. It should also generate mouse models of degenerative joint disease that could be used to test future gene- and cell-based therapies for joints conditions common to osteoarthritic patients and aging individuals.